Glass composite including dispersed rare earth iron garnet nanoparticles

ABSTRACT

Glass/nanoparticle composites are provided which include a glass matrix with a high density of heterologous nanoparticles embedded therein adjacent the outer surfaces of the composite. Preferably, the glass matrix is formed of porous glass and the nanoparticles are yttrium-iron nanocrystals which exhibit the property of altering the polarization of incident electromagnetic radiation; the composites are thus suitable for use in electrooptical recording media. In practice, a glass matrix having suitable porosity is contacted with a colloidal dispersion containing amorphous yttrium-iron nanoparticles in order to embed the nanoparticles within the surface pores of the matrix. The treated glass matrix is then heated under time-temperature conditions to convert the amorphous nanoparticles into a crystalline state while also fusing the glass matrix pores. Nanoparticle loadings on the order of 10 9  nanoparticles/mm 2  of glass surface area are possible, allowing construction of recording media having a recordable data density many times greater than conventional media.

This application is a divisional of U.S. Ser. No. 09/679,856, filed Oct.5, 2000, now U.S. Pat. No. 6,790,521.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is broadly concerned with glass/nanoparticlecomposites, and especially composites containing embedded nanocrystalshaving desirable electrooptical properties allowing the composites to beused in high density recording media. More particularly, the inventionis concerned with such composites, and methods of fabrication thereof,wherein the composites include a glass matrix body with rare earth irongarnet nanoparticles embedded therein. The composites are manufacturedby contacting a suitable porous glass with a colloidal dispersion ofnanoparticles to fill the glass pores, followed by heating of thetreated glass to effect fusing of the glass matrix pores.

2. Description of the Prior Art

Ferromagnetic fine particles such as nanoparticles have attractedconsiderable attention from researchers in recent years. This intereststems from the fact that such particles are single magnetic domainparticles and accordingly their magnetic properties and their mutualinteraction can be studied without magnetic domain effects. Moreover,quantum size effects and the magnetic quantum tunneling effects of theseparticles can be studied because of their nanoparticle dimensions. Froman industrial standpoint, such magnetic particles can be used as mediafor high-density magnetic or magneto-optical information source.

In light of these considerations, efforts have been made in the past toprepare nanoparticle compositions using a variety of different methods.A consistent problem with these prior techniques has been the tendencyof the magnetic nanoparticles to spontaneously coagulate. Thus, theintrinsic magnetic characteristics of the nanoparticles are oftendifficult or impossible to discern even though the nanoparticlecompositions were initially successfully prepared. A number of methodshave been proposed to prevent the coagulation of magnetic nanoparticles,such as to disperse the particles in an organic binder (O'Grady et al.,J. Magn. Mater., 95:341 (1991)), or to disperse the particles in asolvent with an aid of a surfactant (Rosenweig, Ferrohydrodynamics,Cambridge University Press, 1985). These methods use mechanical stirringto disperse the particles, but nevertheless a considerable portion ofthe particles remain coagulated if the particle concentration is high.

In other research, magnetic fine particle precursors have been dispersedin sol-state glass precursor, the magnetic particles were precipitatedin solidified glass or simple magnetic fine particles (e.g., elementaliron or cobalt or simple crystalline structure such as iron oxide) wereintroduced into the pores of porous glass. However, owing to the factthat these techniques involve the precipitation of precursor particlesinto a glass matrix, or ion sputtering on porous glass, it has beendifficult to control the fabrication of the products. As a consequence,these methods have not been applicable using fine particles of complexcrystalline structure.

SUMMARY OF THE INVENTION

The present invention overcomes the problems outlined above and providesnew glass/nanoparticle composites and methods of fabrication thereof,allowing controlled production of very high nanoparticle densitycomposites. Broadly speaking, the composites of the invention include abody of glass having embedded therein a plurality of heterologousnanoparticles, with at least certain of the nanoparticles having adiameter of up to about 500 nm. Preferably, the nanoparticles arecharacterized by the property of altering the polarization of incidentelectromagnetic radiation which is reflected or scattered from thenanoparticles. The most preferred type of nanoparticles are the rareearth iron garnet nanocrystals, especially yttrium-iron nanoparticles.Here, the term “nanoparticle” is defined as a particle, the size ofwhich is between several nanometers and several hundred nanometers. Theterm “nanocrystal” is defined as a crystal grain the size of which isbetween a several nanometers and several hundred nanometers.

In forming the composites, a porous glass body such as “thirsty glass”is contacted with a dispersion including the heterologous nanoparticles,so that such nanoparticles locate within the surface pores of the glassbody. Thereafter, the nanoparticle-treated glass body is heated to fusethe pores and embed the nanoparticles. Most preferably, thenanoparticles are initially in an amorphous state, and the heating stepserves to transform these nanoparticles into a crystalline state. Thesemethods produce stable composites which can be used as a part ofelectrooptical recording media. Nanoparticle loadings on the order of10⁹ nanoparticle/mm² of glass surface area are possible. This allowsconstruction of recording media having a recordable data density manytimes greater than conventional media.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph of the microporous surface ofcontrolled pore glass useful in forming the composites of the invention;

FIG. 2 is a schematic representation illustrating controlled pore glasswith nanoparticles located within the pores thereof to form aglass/nanoparticle composite, both before and after calcination of thecomposite;

FIG. 3 is a graph containing X-ray diffraction (XRD) patterns forindividual samples of large pore (292 nm pore size before calcination)controlled pore glass/nanoparticle composites formed using differentmaximum calcination temperatures with the same holding time (2 hrs),wherein the following legend is applicable: •: Fe₅Y₃CO₁₂, ∘:cristobalite(1), ⊙: cristobalite(2),

: quartz, Δ: Y₂Si₂O₇(1), ▴: Y₂Si₂O₇(2-), ∇: Fe₂SiO₄(1), ▾: Fe₂SiO₄(2),x: Y₂SiO₅, : ⋄—Fe₂O₃, ♦: ε-Fe₂O₃, □: Y₂O₃; cristobalite(1),cristobalite(2) etc. denote polymorphisms which are shown in captions ofTable 1;

FIG. 4 is a graph containing XRD patterns for individual samples ofsmall pore (48.6 nm pore size before calcination) controlled poreglass/nanoparticle composites formed using different maximum calcinationtemperatures with the same holding time (2 hrs), wherein the FIG. 3legend is applicable;

FIG. 5 is a graph containing XRD patterns for individual samples oflarge pore (292 nm pore size before calcination) controlled poreglass/nanoparticle composites formed using different maximum calcinationtemperatures and different holding times, wherein the FIG. 3 legend isapplicable;

FIG. 6 is a graph containing XRD patterns for individual samples ofcontrolled pore glass/nanoparticle composites formed using controlledpore glasses of different pore sizes (114 and 204 nm before calcination,respectively) and with different maximum calcination temperatures andthe same maximum temperature holding time (2 hrs), wherein the followinglegend is applicable;

FIG. 7 is a transmission electron microscope (TEM) photograph of acontrolled porous glass/nanoparticle composite in accordance with theinvention (sample A₁);

FIG. 8 is a TEM photograph of a controlled porous glass/nanoparticlecomposite in accordance with the invention (sample A₂);

FIG. 9 is a TEM photograph of a controlled porous glass/nanoparticlecomposite in accordance with the invention (sample A₃); and

FIG. 10 is a schematic representation of possible physical processesoccurring during calcination of the preferred controlled porousglass/yttrium-iron nanoparticle composites of the invention, including:(a) before calcination, both YIG particles and CPG are amorphous; (b)the crystallization of amorphous YIG particles and amorphous silicate,or CPG; (c) reaction between YIG particles and CPG on the boundary; and(d) decomposition of iron and yttrium silicate to ε-Fe₂O₃ and Y₂O₃ athigh temperature calcination. The circles' radius is proportional tostable iron and yttrium compounds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following examples set forth the production and testing of exemplaryglass/nanoparticle composites in accordance with the invention. It is tobe understood, however, that these examples are provided by way ofillustration and nothing therein should be taken as a limitation uponthe overall scope of the invention.

Preparation Methods

The amorphous YIG nanoparticles were prepared by the alkoxide method.Broadly speaking, in this method the starting materials are solutions ofFe³⁺ alkoxide Fe(OR)₃ and a solution of yttrium alkoxide Y(OR′)₃ where Rand R′ are respectively taken from the group consisting of the alkylgroups. The two solutions are mixed to provide a stoichiometric Fe:Yratio of 5:3, and the mixture is heated to boiling with vigorousstirring. Hot water vapor is introduced into the mixture to causehydrolysis, thereby yielding nanoparticles of the mixture of amorphousiron oxide and yttrium oxide expressed by the equation5Fe(OR)₃+3Y(OR′)₃÷24H₂O[3Y(OH)₃][5Fe(OH)₃](nanoparticles)+15ROH+9R′OH.

Nanoparticles of the chemical formula, Fe₅Y_(3-x)M_(x)O₁₂, orFe₅Y_(3-x-y)M_(x)N_(y)O₁₂ can be prepared according to the reaction

5Fe(OR)₃+(3−x−y)Y(OR′)₃+xM(OR″)_(3—)yN(OR″′)₃+24H₂O[3−x−y)Y(OH)₃][5Fe(OH)₃][xM(OH)₃][yN(OH)₃](nanoparticles)+15ROH+3(3−x−y)R′OH+3R″OH+3yR″′OH,

where, M and N denote either Bi, Gd, In or rare earth elements La, Ce,Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tn, Yb and Ln. x and y denote thenumbers satisfying 0 x÷y 1, R″ and R″′ denote alkyl groups.

In the present case, iron ethoxide and yttrium butoxide were mixed andhydrolyzed to prepare amorphous YIG nanoparticles of 9 nm mean diameter,expressed by the chemical reaction,5Fe(OEt)₃+3Y(OBu)₃+12H₂O [Y₃Fe₅O₁₂](nanoparticles)+15EtOH+9BuOH, whereEt and Bu denote ethyl and butyl groups, respectively.

The primary nanoparticles resulting from this reaction coagulated witheach other to form aggregates or secondary particles. These secondaryparticles were obtained from the reaction mixture by centrifugation.Next, the secondary particles were dispersed in kerosene together witholeic acid surfactant, followed by ball milling. During this process,the secondary particles were broken down and their surfaces coated withsurfactant in order to yield a stable dispersion. If the surfaceparticles from this treatment are not completely covered with surfactantmolecules, a secondary procedure may be followed. After the ball millingstep, additional oleic acid and aqueous ammonia are added to createammonium oleate in the dispersion. This solution is then vigorouslystirred using a homogenizer with heating to a temperature of up to about98° C. The ammonium oleate decomposes to gaseous ammonia and oleic acidabove 78° C. and the isolated oleic acid adhere to the surfaces of thenanoparticles.

The surfactant-coated nanoparticles have a variable size distribution,with the larger particles precipitating in the dispersion, yielding asupernatant containing particles of up to about 30 nm. The supernatantsplit into two layers, water layer (lower) and kerosene layer (upper) inwhich the particles are dispersed. The kerosene-solvent phase wasdecanted and condensed by evaporating part of kerosene solvent.

The porous glass used was controlled porous glass (CPG) prepared by themethod described by Haller: Haller, W., J. Chem Phys, 42:686 (1965);Haller, W., Nature, 286:693 (1965); and Haller, W., Solid PhaseBiochemistry, Chap. 11, Wiley, N.Y. (1983), all of which areincorporated by reference herein. In particular, CPG is manufactured byrapidly cooling a ternary oxide melt (SiO₂—B₂O₃—Na₂O) until itsolidifies into a homogeneous glass. After reheating and annealing atelevated temperature, the glass segregates into two interconnectedphases, an almost pure silica phase and a sodium borate phase havingsome silica dissolved therein. In the final step, the sodium boratephase is removed by acid treatment and silica debris is cleaned from theresultant pores. The CPG contains traces of B₂O₃ and Na₂O which may playa role in the subsequent catalysis of the reaction between SiO₂ and YIGparticles. FIG. 1 is an SEM photograph of a carbon replica of thesurface of typical CPG. In the present examples, four CPG's withdifferent pore sizes were employed, and were obtained from W. Haller ofthe National Institute of Standards and Technology, and from CPG, Inc.of Fairfield, N.J. The mean pore sizes and size distributions were 48.6nm±3.9%, 114 nm±5.2%, 204 nm±11.5% and 292 nm±7.1%. The particle size ofthe 48.6 nm pore size CPG ranged between 75–125 μm while the rest werebetween 37–74 μm.

The CPG particles were soaked in the YIG colloidal dispersion in orderto allow the amorphous YIG particles to pass into the pores of the CPG.This is schematically illustrated in FIG. 2( a). The soaking wasconducted at room temperature for a period of about 24 hrs. Next, thesoaked CPG powder was left in the air and the solvent was evaporated.

After drying, the amorphous nanoparticle-loaded CPG was calcined usingan electric furnace. The temperature was increased from ambient at arate of 200° C. per hour to a maximum temperature T₀ and held at thattemperature for t₀ hrs. The samples were then cooled to roomtemperature. Calcining had the effect of converting the amorphous YIGinto the crystalline particles, and also of fusing the CPG pores so asto fully embed the YIG particles within the glass. (This isschematically illustrated in FIG. 2( b).) The characteristics of thecalcined composites are set forth in Table 1. All composites werecalcined in air except for samples B₃ and D, which were calcined in a N₂gas environment.

TABLE 1 Characteristics of Samples^(a) Prepared compounds Samp. T₀ t₀identified from XRD D A₁ 700 2 Cristobalite(1), Fe₅Y₃O₁₂ 4.2 A₂ 800 2Cristobalite(1), Fe₅Y₃O₁₂ 37 A₃ 850 2 Cristobalite(1), Fe₅Y₃O₁₂, unknownY₂Si₂O₇(1), Fe₂SiO₄(1), Y₂SiO₅ A₄ 900 2 Cristobalite(1), Fe₂O₃,Y₂Si₂O₇(1), None Y₂Si₂O₇(2), Fe₂SiO₄(2), Y₂SiO₅ A₅ 1000 2Cristobalite(1), Fe₂O₃, Y₂Si₂O₇(1), None Y₂Si₂O₇(2), Fe₂SiO₄(2), Y₂SiO₅A₆ 675 16 Cristobalite(1), Fe₅Y₃O₁₂, 11.2 Y₂Si₂O₇(1) A₇ 700 16Cristobalite(1), Fe₅Y₃O₁₂, 22.0 Y₂Si₂O₅(1) A₈ 793 0.1 Cristobalite(1),Fe₅Y₃O₁₂, 37.0 Y₂Si₂O₇(1) A₉ 890 0 Quartz, Fe₅Y₃O₁₂ 41.0 B₁ 700 2Cristobalite(2), Fe₅Y₃O₁₂, Y₂SiO₅ Unknown B₂ 800 2 Cristobalite(1),Fe₂O₃, Y₂O₃ None B₃ 900 2 Cristobalite(1), Y₂Si₂O₇(1), None Y₂Si₂O₇(2),Y₂SiO₅, Fe₂SiO₄(1) B₄ 1200 2 Cristobalite(2), Fe₂O₃, Y₂O₃ None C 800 2Cristobalite(1), Fe₅Y₃O₁₂ Unknown D 900 2 Cristobalite(1), Fe₂O₃,Y₂Si₂O₇(1), None Y₂Si₂O₇(2) ^(a)T₀: calcination temperature (C.), t₀:calcination time (h), D: YIG nanocrystal's mean diameter (nm), sampleA₁(I = 1–9); CPG pore size before calcination 292 nm, sample B₁(i =1–4); CPG pore size before calcination 48.6 nm, sample C: CPG pore sizebefore calcination 204 nm, sample D: CPG pore size before calcination114 nm. The compounds were identified according to the Powder Data Fileof Joint Committee on Powder Diffraction Standard (JCPDS). The compoundscorrespond to the JCPDS number, respectively. Cristobalite(1): 39-1425,cristobalite(2): 76-0936, quartz:93-2465, Fe₅Y₃O₁₂: 43-0507, Y₂Si₂O₇(1):45-0042, Y₂Si₂O₇(2); 21-1459, Fe₂SiO₄(1): 71-1667, Fe₂SiO₄(2): 72-0297,Y₂SiO₅: 21-1461, —Fe₂O₃: 80-2377, —Fe₂O₃: 16-0653, Y₂O₃: 44-0399.Sample Characterization

1. Transmission Electron Microscope Analysis. The calcined compositeswere ground in an agate mortar to reduce the grain size from about 50 μmto submicron size. The milled powder was mostly pure silica without YIGparticles, because the YIG particles in the original CPG grains wereconcentrated primarily in a 0.5 μm subsurface layer. Significant colorchanges were observed after removing this subsurface layer. For example,sample B₄ was tinged with deep red color due to the presence of —Fe₂O₃which was formed by the decomposition of YIG particles duringcalcination. After grinding, the color changed to white pink because ofwhite cristobalite in the core of CPG grains revealed by the grinding.The ground powder was put in a vial filled with 97% ethanol and stirredstrongly. Before all the fragments deposited in the bottom of the vial,the supernatant liquid was removed so that excess pure silica fragmentswere removed. The specific gravity of CPG fragments which contained YIGnanocrystals was greater than that of pure CPG fragments. In addition,the former fragments were easy to coagulate due to the YIG magneticattraction. Accordingly, the CPG fragments with YIG nanocrystalsdeposited faster than the pure CPG fragments. Taking advantage of thisphenomena, the CPG fragments were separated. This process was repeatedseveral times. The liquid was stirred strongly and a droplet of theliquid was put on polymer film of a copper mesh TEM slide, and the CPGfragments were observed by two TEM's, a Philips 201 and a CM100,operating at 100 kV. Representative TEM photographs for samples A₁–A₃are set forth in FIGS. 7–9.

2. X-Ray Diffraction Analysis. Phase composition of the ground sampleswhich were prepared for TEM observation was investigated using XRD(XDS2000 Scintag Inc.). To increase the signal/background ratio from YIGparticles, the slit width of the diffractometer was increased five timesand the measuring time was increased 20 times in comparison with theordinary slit width and the measuring time, respectively. Despite that,the diffraction intensity from YIG particles was very low. Thediffraction intensity increased with angle decrease due to the smallangle peak of the polymer substrate. The diffraction peak width wideningdue to the slit width was compensated for by calibration using standardmicron-size multicrystalline quarts. Using the half value width ofdiffraction peaks, Δ and Δ_(st) for the YIG nanocrystals of the presentsamples and the standard quartz sample, respectively, the meancrystalline size, D, of YIG nanoparticles in the silica matrix wasdetermined by

$D = \frac{0.9\lambda}{\left( {\Delta - \Delta_{st}} \right)\cos\;\theta}$where λ—1.54 Å is the X-ray wavelength and 2 is the diffraction angle.Results and Discussion

FIG. 7 is the TEM photograph of sample A₁ calcined for 2 hrs at 700° C.The small black dots in the silica matrix are the crystalline YIGparticles as indicated by XRD in the lower part of FIG. 3. Largeparticles are pure silica fragments. In FIG. 7, there are 3–5 particlesin a square of 100 nm side. It means that the particles number densityof 0.3×10⁹ to 0.5×10⁹ per square mm of body surface. The meltingtemperature of CPG is between 900 and 1000 C. The region with YIGparticles easily melted or softened at less than 700 C., presumablybecause the YIG particles served as contaminants and promoted poreclosure of CPG. Using selected area electron diffraction (SAED)techniques, it was concluded that a very small portion of the matrixsilica was crystallized but that the amount of crystallized silica, orcristobalite, was so small that the electron diffraction lines ofcristobalite could not be detected in the TEM analysis.

FIG. 8 is a TEM photograph of sample A₂. From this analysis,cristobalite and YIG were identified from the SAED patterns, which isconsistent with the result of XRD analysis of FIG. 5.

FIG. 9 is a TEM photograph of sample A₅. The nanoparticles (black dots)are dispersed and embedded in the silica matrix.

FIG. 3 illustrates XRD patterns from the samples after calcination atthe temperatures from 700° C. to 1000° C. (samples A₁–A₅). The pore sizeof CPG before calcination was 292 nm and the calcination time, t₀, was 2hrs for all the samples. The XRD curves are all shown without smoothingtreatment.

In order to estimate YIG crystal sizes, YIG crystals were prepared aloneusing the corresponding calcination temperatures. The means crystallinesizes D, of the YIG nanoparticles were estimated by the half valuewidth, Δ, of the (420) peak (2=32.314) and Δ_(st) of the (112) peak(2=50.138) of standard multicrystalline quartz, using the above formula.The D value for each sample is shown in Table 1. YIG's D increasesrapidly with the calcination temperature.

Many iron silicate and yttrium silicate compounds were generated by thereaction of YIG particles and CPG or silicate, particularly the samplesof T₀ above 850° C. The identified products are tabulated in Table 1. Inaddition, there are a few peaks which were not be identified. There areseveral polymorphisms for the same chemical formula of iron silicate andyttrium silicate. Therefore, it may be that several polymorphisms ofiron silicate and yttrium silicate were generated in the contact regionbetween YIG particles and CPG.

FIG. 4 shows the XRD patterns of the samples of T₀ at 700° C. and up to1200° C. with small CPG pores (samples B₁–B₄). The calcination time t₀was 2 hrs and the mean pore diameter before calcination was 48.6 nm forall the samples. All the samples were calcined in the air except sampleB₃ which was calcined in a N₂ gas environment. The YIG particles weredecomposed to different compounds for the samples of T₀ at 800° C. Inaddition, the iron and yttrium silicate disappeared and YIG particlesdecomposed almost to ε-Fe₂O₃ and Y₂O₃ for the sample of T₀—1200° C.

FIG. 5 shows XRD patterns of the samples for different calcination time,t₀ from 0 to 16 hrs (samples A₆–A₉) using different T₀ values. In thiscase of sample A₉, when the furnace temperature reached 890° C., it wasimmediately turned off and the sample was allowed to cool in the furnaceto ambient. From the results of sample A₆, it was found that YIGparticles and silicate reacted to form iron and yttrium silicate duringthe extended heat treatment, even at a low-temperature calcination of675° C. This indicates that iron or yttrium silicates are more stablethan the separated state of YIG and silica. The formation of the ironand yttrium silicate at such a low temperature may also indicate thatthe contaminants, B₂O₃ and Na₂O in CPG played a role as catalysts forthe reaction of YIG particles and CPG to form yttrium and iron silicate.

FIG. 6 shows the XRD results for the samples of pore size 204 and 114 nmbefore calcination (samples C and D). Sample D was calcined in a N₂ gasenvironment. The B₂ and D samples showed the preserve of ε-Fe₂O₃, anuncommon and special oxide.

It is believed that three different processes occur in parallel duringcalcination to form the composites of the invention. FIG. 10 includesschematic representations of these processes. FIG. 10 a is the statebefore the calcination. Both the particles and the CPG are in anamorphous phase. The radius of the circles which denote oxygen, iron,yttrium and silicon ions, respectively, are proportional to each ion'sradius. The first process is the phase transition of YIG particles fromamorphous to crystalline state. The second process is the phasetransition of CPG from amorphous to cristobalite crystal. These twoprocesses are shown in FIG. 10 b. The third process is the reaction ofYIG particles and contact with CPG to generate iron silicate and yttriumsilicate, which is schematically shown in FIG. 10 c. The third processproceeds slowly compared with the first and second processes inlow-temperature calcinations. Accordingly, it is believed that only thefirst and second processes proceed in low temperature-short timecalcinations. The CPG of small pore size has large surface areacontacting the YIG particles, and accordingly the third process proceedseven in low temperature-short time calcination. In addition, in hightemperature calcinations as high as 1200° C., the yttrium and ironsilicate transformed to more stable compounds ε-Fe₂O₃, Y₂O₃, and SiO₂(see FIG. 10 d).

In conclusion, for the purpose of preparing YIG nanocrystals dispersedin silica glass, the calcination should most preferably be carried outby increasing the temperature as rapidly as possible and immediatelyafter the temperature reaches a temperature near 900° C., at which theamorphous YIG particles are crystallized, the temperature should bedecreased in order to minimize the reaction between YIG and silica.

While the foregoing examples set forth preferred methods andglass/nanoparticle composites, the invention is not so limited. Forexample, the invention broadly involves the embedment of heterologousnanoparticles (i.e., nanoparticles chemically different from theconstituents of the glass matrix) of any physical type (for example,amorphous or crystalline, in the latter case as single crystals orcrystallites), so long as the particles have a maximum diameter of up toabout 500 nm, more preferably of up to about 300 nm, and most preferablyfrom about 10–150 nm. The preferred nanoparticles are rare earth irongarnet nanoparticles, especially yttrium-iron nanoparticles. In thelatter case, nanoparticles having the formula Fe₅Y_(3-x-y)M_(x)N_(y)O₁₂where M and N are different and are respectively taken from the groupconsisting of Bi, Gd, In, La, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tn, Yband Ln, and x and y are selected to satisfy the equation 0≦x+y≦1 can beused to good effect.

The glass component of the composites of the invention is preferablymade up of a porous glass, especially “thirsty glass.” The latter typeof material is described in U.S. Pat. Nos. 4,503,257 and 4,842,968 (bothof which are incorporated by reference herein) as well as the Hallerreferences previously cited. The starting porous glass should have apore diameter of up to about 15–400 nm, and more preferably up to about50–300 nm. After the calcining step, the pores of the glass are fused toenvelope and embed the nanoparticles therein.

The preferred composites have a relatively high density of nanoparticlesembedded therein. Advantageously, the nanoparticles should be present ata level of from about 10–10⁹ nanoparticles per mm² or glass bodysurface, and more preferably from about 10⁷–10⁹ nanoparticles per mm² orbody surface. If we assume uniform dispersion of the particles in thesurface layer, the particles number densities per square nm isequivalent with the following conditions. Advantageously, thenanoparticles should be present at a level of from about 10³−3×10¹³ percubic mm of glass body surface layer and more preferably from about3×10¹⁰ to 3×10¹³ per cubic mm of the glass body surface layer.

The composites find particular utility as electrooptical recordingmedia. This stems from the fact that the nanoparticles have an importantelectrooptical property. Specifically, when the nanoparticles aresubjected to incident electromagnetic radiation of known polarization,the reflected or scattered radiation from the particles has a changedpolarization. This allows one bit of memory to be assigned to eachindividual nanocrystal. Thus, the composites can be used as anefficient, high density recording medium.

In the manufacture of the composites, starting porous glass bodies areprovided. The glass bodies are preferably contacted with a dispersionincluding the heterologous nanoparticles, so as to cause at leastcertain of the latter to locate within the pores of the glass.Thereafter, the dispersion-treated glass is subjected to heating(calcining) in order to fuse the pores and thus fully embed thenanoparticles located therein. Preferably, when the yttrium-iron garnetnanoparticles are used, the heating process also has the effect ofconverting the nanoparticles in situ from an amorphous to a crystallinestate.

The heating or calcining step is of importance in determining thecharacteristics of the final composite product. Generally speaking, theheat should be conducted so that the amorphous YIG nanoparticles isheated to a maximum temperature of at least about 650° C. for a periodof time to effect the amorphous YIG crystallizing. The preferabletemperature depends on the period of time. For example, if we adopt 2hrs of period of time, the temperature is more preferable from 650–900°C. and most preferably about 800° C. On the other hand, if the period oftime is 0, the temperature is preferably from 850–950° C. and mostpreferably about 900° C. In addition, for the latter case, i.e., 0 hourperiod of time, the heating rate should be at least about 100° C. perhour more preferably faster than 200° C. per hour and after reaching themaximum temperature, sample should be cooled faster than 100° C. perhour and preferably faster than 200° C. per hour.

In conclusion for the period time ranging from 0.5 hr to 20 hrs, thetemperature is more preferable from 650–900° C. and most preferableabout 800° C. For the period time ranging from 0 to 0.5 hrs, thetemperature is preferably from 850–950° C. and most preferably about900° C. with extra conditions of the heating rate at least about 100° C.per hour, and more preferably faster than 200° C. per hour and afterreaching the maximum temperature, cooling rate faster than 100° C. perhour and preferably faster than 200° C. per hour.

1. A method of forming a composite comprising of the steps of: providinga porous glass body; contacting said body with a dispersion includingamorphous nanoparticles, and causing at least some of said nanoparticlesto locate within pores of said body; and fusing said pores to embed saidnanoparticles located in said body; wherein said fusing step comprisesthe step of heating said body by progressively increasing thetemperature of said body to a maximum temperature of about 650° C. toabout 900° C. for a time period of about 0.5 to about 20 hours, wherebysaid fusing is effected and said amorphous nanoparticles are convertedinto nanocrystals.
 2. The method of claim 1, said contacting stepcomprising the step of forming a colloidal dispersion of saidnanoparticles, and soaking said body in said colloidal dispersion. 3.The method of claim 1, said maximum temperature being about 800° C. 4.The method of claim 1, including the step of progressively increasingthe temperature of said body to said maximum heating temperature,keeping the temperature for a period of time ranging from 0 to 0.5hours, followed by cooling of the body to ambient temperature.
 5. Themethod of claim 4, said maximum temperature being from about 850° C. to950° C.
 6. The method of claim 5, said maximum temperature being about900° C.
 7. The method of claim 4, including the step of heating saidbody at a rate of at least about 100° C. per hour until said maximumheating temperature is reached.
 8. The method of claim 7, said ratebeing at least about 200° C. per hour or faster.
 9. The method of claim4, including the step of cooling said body at a rate of at least about100° C. per hour until ambient temperature is reached.
 10. The method ofclaim 9, said rate being at least about 200° C. hour or faster.
 11. Themethod of claim 1, said nanoparticles having a diameter of up to about300 nm.
 12. The method of claim 1, there being at least about 10–10⁹nanocrystals per mm² of a surface of said body.
 13. The method of claim12, said nanocrystals being present at a level of from about 10⁷–10⁹nanocrystals per mm² of said body surface.
 14. The method of claim 1,there being at least about 10³−3×10¹³ nanoparticles per mm³ of surfacelayer of said body.
 15. The method of claim 14, said nanoparticles beingpresent at a level of from about 3×10¹⁰–3×10¹³ nanoparticles per mm³ ofsaid surface layer of said body.
 16. The method of claim 1, at leastcertain of said nanoparticles within said dispersion being yttrium-irongarnet nanoparticles.
 17. The method of claim 1, said nanocrystals beingrare earth iron garnet nanocrystals.
 18. The method of claim 17, saidrare earth iron garnet nanocrystals having the formula ofFe₅Y_(3-x-y)M_(x)N_(y)O₁₂ where M and N are different and arerespectively taken from the group consisting of Bi, Gd, In, La, Pr, Nd,Pm, Sm, Eu, Tb, Dy, Ho, Er, Tn, Yb and Ln, and x and y are selected tosatisfy the equation of 0≦x+y≦1.
 19. The method of claim 1, said porousglass body being thirsty glass.
 20. The method of claim 1, saidnanoparticles being formed by the alkoxide method.
 21. The method ofclaim 1, said dispersion comprising a kerosene and surfactant mixture,said dispersion formed by agitating said nanoparticles in said mixtureso as to coat said nanoparticles with said surfactant.
 22. A method offorming a composite comprising of the steps of: providing a porous glassbody; contacting said body with a dispersion including amorphousnanoparticles, and causing at least some of said nanoparticles to locatewithin pores of said body; and fusing said pores to embed saidnanoparticles located in said body; wherein said nanoparticles areconverted to yttrium iron garnet nanocrystals having the formulaFe₅Y₃O₁₂.
 23. The method of claim 22, wherein said fusing step comprisesthe step of heating said body by progressively increasing thetemperature of said body to a maximum temperature of about 650° C. toabout 900° C. for a time period of about 0.5 to about 20 hours, wherebysaid fusing is effected and said amorphous nanoparticles are convertedinto nanocrystals.
 24. The method of claim 22, said contacting stepcomprising the step of forming a colloidal dispersion of saidnanoparticles, and soaking said body in said colloidal dispersion. 25.The method of claim 23, said maximum temperature being about 800° C. 26.The method of claim 23, including the step of progressively increasingthe temperature of said body to said maximum heating temperature,keeping the temperature for a period of time ranging from 0 to 0.5hours, followed by cooling of the body to ambient temperature.
 27. Themethod of claim 26, said maximum temperature being from about 850° C. to950° C.
 28. The method of claim 27, said maximum temperature being about900° C.
 29. The method of claim 26, including the step of heating saidbody at a rate of at least about 100° C. per hour until said maximumheating temperature is reached.
 30. The method of claim 29, said ratebeing at least about 200° C. per hour or faster.
 31. The method of claim26, including the step of cooling said body at a rate of at least about100° C. per hour until ambient temperature is reached.
 32. The method ofclaim 31, said rate being at least about 200° C. per hour or faster. 33.The method of claim 22, said nanoparticles having a diameter of up toabout 300 nm.
 34. The method of claim 22, there being at least about10–10⁹ nanocrystals per mm² of a surface of said body.
 35. The method ofclaim 34, said nanocrystals being present at a level of from about10⁷–10⁹ nanocrystals per mm² of said body surface.
 36. The method ofclaim 22, there being at least about 10³−3×10¹³ nanoparticles per mm³ ofsurface layer of said body.
 37. The method of claim 36, saidnanoparticles being present at a level of from about 3×10¹⁰–3×10¹³nanoparticles per mm³ of said surface layer of said body.
 38. The methodof claim 22, said porous glass body being thirsty glass.
 39. The methodof claim 22, said nanoparticles being formed by the alkoxide method. 40.The method of claim 22, said dispersion comprising a kerosene andsurfactant mixture, said dispersion formed by agitating saidnanoparticles in said mixture so as to coat said nanoparticles with saidsurfactant.